The present invention relates generally to sensing down hole conditions in an oil well drill hole, and the like, and more particularly to an instrument system for measuring pressure and temperature conditions within the well as a function of time and depth within the hole, and for recording the data.
The importance of obtaining down hole pressure and temperature readings has long been recognized in the petroleum industry. For example, the presence and thickness of oil and water layers below the surface can be determined and distinguished by the pressure gradient existing between different depths along the well hole. The rate of decrease in pressure at the bottom of the well with time when oil flows, and the rate of recovery in pressure with time when flow is terminated is indicative of the geological characteristics of the oil bearing structure.
In the prior art down-hole gauges have relied on electromechanical systems to record the desired data. Well pressure is frequently sensed via the expansion and contraction of a Bourdon tube and data is recorded on a moving chart using a stylus engaged by a linkage mechanism, or directly, to a moving end of the Bourdon tube. Uniform chart motion and accurate stylus response are requisites. The mechanical systems tend to be large and intricate and the timing motor for the chart drive causes a continuous and substantial drain on the batteries which are part of the underground apparatus. Because the diameter of the guage is limited by the drill tubing diameter, the length of the gauges is large, in the order of 6 feet. With modern production techniques, where the tubes curve, and may bend with a radius as small as 5 feet, instruments of extended length are inapplicable.
Provision must also be made in the design to retrieve the chart, which may be flat or on a cylindical drum, from the mechanical mechanism and to insert a replacement. The analog trace from the chart is then manually interpreted and re-recorded in terms of the desired parameters, e.g., pressure and temperature.